1. Field of the Invention
The present invention relates to an LPG (liquefied petroleum gas) engine using LPG as fuel. Particularly, the present invention relates to a fuel control apparatus for controlling LPG that is to be fed into an LPG engine in an idle operation.
2. Description of the Related Art
Conventionally, there is an LPG engine of the aforementioned type that operates using LPG as fuel which is decompressed by an LPG regulator and which is fed into a carburetor and an injector provided in each intake passage. The LPG regulator decompresses high-pressure LPG from an LPG tank at two steps, and feeds the decompressed LPG through the carburetor into the intake passage as primary fuel according to a venturi vacuum. The LPG regulator also feeds decompressed LPG as supplementary fuel into the injector. On the basis of an air/fuel ratio detected by an oxygen sensor provided in an exhaust passage, a controller feedback-controls the amount of the supplementary fuel that is to be injected from the injector into the intake passage. Specifically, in the LPG engine, the supplementary fuel fed from the injector is added to the primary fuel fed from the carburetor, and the controller controls the feed amount of the supplementary fuel fed from the injector to cause an air/fuel ratio of the two types of fuel and air to be a predetermined stoichiometric air/fuel ratio.
A slow passage is provided in the LPG regulator. In an idle operation of the LPG engine, part (slow fuel) of LPG decompressed by the LPG regulator passes through the slow passage when flowing into the carburetor as the primary fuel. As such, the primary fuel to be fed into the intake passage during the idle operation varies depending on the slow-fuel flow rate.
An adjusting screw is provided in the slow passage. The adjusting screw is operated to adjust the slow-fuel flow rate in order to adjust the air/fuel ratio (idle air/fuel ratio) in the idle operation. During the idle operation, an operator monitors a control variable or a correlation value thereof that has been output from the controller through a measuring instrument, such as a tester, connected to a predetermined monitor terminal. While performing the monitoring, the operator operates the adjustment screw to set the control variable or the like to a predetermined target value. According to the operation, the controller adjusts the idle air/fuel ratio to the predetermined stoichiometric air/fuel ratio. That is, the operator operates the adjustment screw by referencing the tester or the like and monitoring whether the amount of the supplementary fuel fed from the injector is greater or less than a predetermined set value. Through the above operation, the ratio between the primary fuel (slow fuel) and the supplementary fuel that are to be fed into the intake passage at the time of the idle operation is approximated to a predetermined value. Consequently, the idle air/fuel ratio is adjusted to the stoichiometric air/fuel ratio.
FIG. 11 is a graph showing the relationship between the ratio between the primary fuel and the supplementary fuel and an air/fuel ratio A/F. As shown in FIG. 11(a), the primary fuel and the supplementary fuel are fed at a predetermined ratio so as to adjust the air/fuel ratio A/F to the stoichiometric air/fuel ratio. In this particular case, the primary fuel that has been fed from the carburetor and that contributes to the adjustment in the air/fuel ratio A/F on the lean side is up to a predetermined air/fuel ratio (for example, xe2x80x9c17xe2x80x9d), and the supplementary fuel from the injector is feedback-controlled for the rest. Consequently, the air/fuel ratio A/F can be approximated to the stoichiometric air/fuel ratio. If no change with a lapse of time occurs in the function of the LPG regulator, the above-described relationship is established.
However, with the above-described conventional LPG regulator, a case can occur where, because of change with a lapse of time, tar adheres and accumulates in a gap between the adjustment screw and the slow passage. When tar adheres and accumulates therein, a reduction occurs in slow fuel flowing through the slow passage into the carburetor during the idle operation, thereby causing the idle air/fuel ratio to vary to the lean side. In this case, as shown in FIG. 11(b), the relationship in the ratio between the primary fuel and the supplementary fuel is disordered by the amount of reduction in the primary fuel, and the ratio in which the primary fuel contributes to the air/fuel ratio A/F tends to be lower than xe2x80x9c17xe2x80x9d on the lean side (lean tendency). For the rest, by increasing the supplementary fuel from a predetermined amount Q1 shown in FIG. 11(a) to a predetermined amount Q2 shown in FIG. 11(b), the idle air/fuel ratio can be approximated to the stoichiometric air/fuel ratio.
However, when the aforementioned condition further proceeds, the primary fuel further decreases, and the supplementary fuel further increases. In this case, since the ratio of the primary fuel is low, the feed amount of the primary fuel needs to be increased, and the feed amount of the supplementary fuel needs to be returned to the predetermined amount Q1. To achieve the above, the slow-fuel flow rate should be adjusted by using the adjustment screw. However, since a primary pressure in the LPG regulator and a venturi vacuum are high, even when the adjustment screw is slightly adjusted, the fuel amount tends to sensitively increase. This causes problems in that adjustment for the idle air/fuel ratio becomes difficult, taking a longer time than required for the adjustment.
The present invention is made in view of the above situation, and has an object to provide an LPG-engine fuel control apparatus that securely adjusts a slow-fuel flow rate in an LPG regulator, thereby enabling an idle air/fuel ratio to be suitably and quickly adjusted.
To achieve the above described object, there is provided an LPG-engine fuel control apparatus arranged such that LPG decompressed by an LPG regulator is fed as primary fuel into an LPG engine from a carburetor through an intake passage; the LPG decompressed is fed as supplementary fuel for supplementing the primary fuel from supplementary-fuel feeding means into the LPG engine through the intake passage; an air/fuel ratio between the primary fuel and the supplementary fuel and air that are fed is detected by air/fuel-ratio detecting means; the supplementary-fuel feeding means is controlled according to a required control variable to cause the detected air/fuel ratio to be a predetermined stoichiometric air/fuel ratio; and in an idle operation of the LPG engine, an optimum state of the control variable with respect to a predetermined target value is output from monitoring output means and monitored, slow-fuel adjusting means provided in a slow passage of the LPG regulator is operated to cause the control variable to be the target value, and an idle air/fuel ratio is thereby adjusted to the stoichiometric air/fuel ratio, the LPG-engine fuel control apparatus including: range-setting means to be operated to set an adjustment range of the control variable to multistage adjustment ranges each having a different width including the target value; and output control means for controlling an output pattern of the monitoring output means in each of the adjustment ranges set by the range-setting means, according to whether or not the control variable falls within the adjustment range.
According to the above configuration, in the idle operation of the LPG engine, the range-setting means is operated by an operator, and the adjustment range of the control variable is narrowed and set in stages, for example, from a first stage to a subsequent stage. In each of the stages, the state of appropriateness of the control variable with respect to the target value is output from the monitoring output means and is monitored. Through the monitoring, the slow-fuel adjusting means is operated to cause the control variable to be the predetermined target value. At this time, adjustment is performed for the amount of primary fuel that is to be fed from the carburetor into the intake passage, and the control variable for the supplementary-fuel feeding means can be changed to adjust the feed amount of the supplementary fuel according to the adjusted amount of primary fuel. At this stage, the output pattern of the monitoring output means is controlled by the output control means according to whether or not the control variable that changes through operation of the slow-fuel adjusting means falls within the adjustment range that has been set. That is, the output pattern of the monitoring output means can be changed depending on whether or not the control variable falls within the set adjustment range.
As such, in the first stage, the operator is permitted to operate the slow-fuel adjusting means for a coarse adjustment so that the control variable falls within the adjustment range set to be relatively wide. The adjusting operation enables the control variable to approximate substantially to be the target value. In the subsequent stage, the operator operates the slow-fuel adjusting means to finely adjust it in a step-by-step manner so that the control variable falls within the adjustment range set to be gradually narrowed. The adjusting operation causes the control variable already approximated substantially to be the target value to be set step by step to the target value.